var arr=['html','css','javascript']
console.log(arr.indexOf('css'))
var app=[1,2,3,4,5,6,7];
console.log(app.slice(0,3))
// slice(index,number) 查询前几个
// 查询某个的下标
// includes() 判断数组是否包含某个值
console.log(arr.includes('cs'));